1. Field of Invention
The present invention relates to semiconductor transducers and more particularly to an apparatus and method of compensating the output of the transducer for both temperature and pressure effects by using both discrete electrical elements and a microprocessor.
2. Description of the Related Art
The silicon pressure transducer is a well known device that has been used for many years. The assignee of the present invention holds many patents such as U.S. Pat. No. 6,229,427, entitled “Covered Sealed Pressure Transducers and Method for Making the Same” and U.S. Pat. No. 6,272,929, entitled “High Pressure Piezoresistive Transducer Suitable for use in Hostile Environments” that are directed at the construction of such transducers. These devices are by themselves highly accurate at room temperature, but changes in temperature can affect the electrical characteristics inducing errors into any measurements done at variable temperatures or non-optimal temperatures. Although the new leadless design is more accurate with temperature changes, it is still desirable to correct for temperature fluctuations. There are several known ways of compensating for these errors in temperature measurements. One such way is by using discrete resistors such as in U.S. Pat. No. 3,245,252, entitled “Temperature Compensated Semiconductor strain Gauge Unit”. In this method the changes in resistance and sensitivity with temperature are used together to correct the output. Another method of compensating a transducer for temperature variations is by using discrete active semiconductor components such as op-amps to correct for pressure non-linearities. A method of this type is disclosed in U.S. Pat. No. 4,419,620, entitled “Linearizing Circuits for a Semiconductor Pressure Transducer”.
In recent years microprocessors have been used to correct for errors arising from changes in temperature. While such uses were envisioned quite some time ago, as for example in U.S. Pat. No. 4,192,005, entitled “Compensated Pressure Transducer Employing Digital Processing Techniques”, it is only recently that it has become practical to use such devices on a large scale. In such devices a Wheatstone bridge type pressure transducer is hooked up directly to a microprocessor chip. The microprocessor then uses an internal or external temperature sensor to sense the temperature of the bridge. It then uses internally stored constants to correct the output of the bridge such that the output stays relatively constant over the whole temperature range. Ideally with such a compensation scheme it would be possible to produce a very stable output over a large temperature range.
However there are limitations to what microprocessor correction can do. Standard compensation microprocessors are programmed at several temperature points and then use this information to correct linearly between the given temperature points, in that way it is possible to correct a wide range of temperatures without having to do an excessive amount of testing at temperature before hand. The problem arises when the transducers output does not change linearly between the temperature chosen. In this case errors result which are proportional to the non-linearities between the points chosen for compensation. Another limitation of the current microprocessors is that when a low output transducer is used with one the amplifier must be set at a very high gain; this high gain can make it more difficult to compensate the transducer very accurately; therefore currently, transducers must be chosen that have higher outputs and such transducers are often more non-linear with pressure. The proposed invention is a novel method of combining various compensation schemes together to more effectively and more easily compensate a silicon pressure transducer.